Radiographic image represented by an X-ray image is utilized in wide applications such as for disease diagnosis. So-called radiography is mainly used as the method of obtaining an X-ray image, where the radiation through an object is irradiated on a fluorescent layer (which is also called a fluorescent screen), the visible light generated on the fluorescent layer is then irradiated on a silver halide photosensitive material (hereinafter also called photosensitive material), and a visible image is obtained through development. Recently, however, a new method of obtaining image directly from the fluorescent layer has been proposed instead of the image forming method using photosensitive material containing silver halide.
With this method, the radiation through an object is absorbed into a phosphor and then, by exciting the phosphor by optical or thermal energy for example, the radiation energy stored in the phosphor through the absorption of X-ray is irradiated as fluorescence, and the fluorescence is detected to form an image. To be concrete, it is a radiation image conversion method using stimulable phosphor as disclosed for example in the U.S. Pat. No. 3,859,527 and Japanese Patent Publication Open to Public Inspection (hereafter referred to as JP-A) No. 55-12144 (1980).
This method employs a radiation image conversion panel containing stimulable phosphor; to speak in detail, the radiation through an object is irradiated on the stimulable phosphor layer of the radiation image conversion panel so as to store radiation energy corresponding to the radiation transmission density of each portion of the object and then, by exciting the stimulable phosphor by electromagnetic wave (exciting light) such as visible ray or infrared ray, the radiation energy stored in the stimulable phosphor is emitted as stimulable light emission, and the signal of the intensity of this light is outputted for example as a photoelectrically converted electric signal so as to reproduce a visible image on an existing image recording material such as photosensitive material or on an image display such as CRT.
The above method of reproducing the radiographic image record has an advantage over the conventional radiography using a combination of radiographic photosensitive material and sensitized paper that the radiographic image containing plenty of data volume can be obtained under far less dose.
On the surface of the stimulable phosphor layer used in this technique (surface not facing the substrate), there is normally provided a protection layer for protecting the phosphor layer from chemical decomposition or physical impact. Well-known protection layers are formed as follows: (i) transparent organic polymer such as cellulose derivative or polymethyl methacrylate is dissolved into suitable solvent and then the prepared solution is applied on the phosphor layer; (ii) a protection layer forming sheet or film is separately made using organic polymer film, such as polypropylene or polyethylene terephthalate, or glass plate and then bonded on the surface of the phosphor layer with suitable adhesive; and (iii) an inorganic compound is formed into a film on the phosphor layer such as by vapor deposition.
While the stimulable phosphor is a phosphor that causes stimulable light emission by irradiating exciting light after it is irradiated with radiation as explained above, a phosphor that causes the stimulable light emission in the wavelength ranging from 300 to 500 nm at the exciting light in the wavelength ranging from 400 to 900 nm is generally employed in practice. Well-known stimulable phosphors used conventionally for the radiation image conversion panel include for example rare-earth element activated alkaline-earth metal halide fluoride type phosphor as disclosed in JP-A Nos. 55-12145, 55-160078, 56-74175, 56-116777, 57-23673, 57-23675, 58-206678, 59-27289, 59-27980, 59-56479, and 59-56480; divalent europium activated alkaline-earth metal halide type phosphor as disclosed in: JP-A Nos. 59-75200, 60-84381, 60-106752, 60-166379, 60-221483, 60-228592, 60-228593, 61-23679, 61-120882, 61-120883, 61-120885, 61-235486, and 61-235487; rare-earth element activated oxyhalide phosphor as disclosed in the JP-A No. 59-12144; cerium activated trivalent metal oxyhalide phosphor as disclosed in JP-A No. 58-69281; bismuth activate alkaline metal halide type phosphor as disclosed in the JP-A No. 60-70484 (1985); divalent europium activated alkaline-earth metal halo-phosphate phosphor as disclosed in JP-A Nos. 60-141783 and 60-157100; divalent europium activated alkaline-earth metal halo-borate phosphor as disclosed in the JP-A No. 60-157099; divalent europium activated alkaline-earth metal-hydride halide phosphor as disclosed in the JP-A No. 60-217354; cerium activated rare-earth composite halide phosphor as disclosed in the JP-A Nos. 61-21173 and 61-21182; cerium activated rare-earth halo-phosphate phosphor as disclosed in the JP-A No. 61-40390; divalent europium activated halide cerium/rubidium phosphate as disclosed in the JP-A No. 60-78151; divalent europium activated halogen phosphor as disclosed in the JP-A No. 60-78153; and 14-hedral rare-earth metal activated alkaline-earth metal halide fluoride type phosphor as disclosed in the JP-A No. 7-233369.
Of the stimulable phosphors mentioned above, divalent europium activated alkaline-earth metal halide fluoride type phosphor containing iodine, divalent europium activated alkaline-earth metal halide type phosphor containing iodine, rare-earth element activated rare-earth oxyhalide type phosphor containing iodine, and bismuth activate alkaline metal halide type phosphor containing iodine exhibit high luminance stimulable light emission.
Because the radiation image conversion panel using the above stimulable phosphors emits stored energy when scanned with exciting light after the radiographic image data has been stored, it has an advantage that the radiographic image can be stored again after scanning and so the image can be utilized repeatedly. In other words, while radiographic photosensitive material is consumed in every photo shooting in the conventional radiography, the radiation image conversion panel is utilized repeatedly in this radiation image conversion method, and so it is advantageous in view of resource preservation and economical efficiency.
Although the method of reproducing radiographic image record has a lot of advantages as explained above, the radiation image conversion panel used in this method is desired to provide images with as high sensitivity and high quality (such as sharpness and grainness) as possible.
If the radiation image conversion panel, which is required of high image quality as above, is used for a long time, there arises a problem that foreign substance such as fine dust is collected on it due to charging and the foreign substance causes foreign-substance defect on the image.
Generally, as a way for preventing the collection of foreign substance due to charging as explained above, lowering the surface resistivity is well known. A concrete means for this has been such that metal powder, carbon black or charge transfer complex is mixed in the material or applied as a coating layer so as to add conductivity to the material. This, however, is not good for practical use in view of transparency that is a fundamental characteristic of image forming material and mechanical strength that is required for protection layer material.
A further improved way is such that a coating layer containing more transparent stuff such as surface active agent or metal oxide is provided on the surface of the material as antistatic agent.
However, since surface active agent relies highly upon humidity, it cannot exhibit sufficient antistatic capability under low humidity.
In addition, in order for both surface active agent and metal oxide to add sufficient antistatic capability, that is, to allow low surface resistivity when employed in a product, they must exist on the very top surface. This, however, may deteriorate the moisture resistance that is required for a protection layer, and so a method to retain the moisture resistance may be employed when utilized in practice. (See the Patent Documents 1 and 2, for example.)
An example of a radiation image conversion panel composed of a stimulable fluorescent screen is disclosed in Patent Document 3. The radiation image conversion panel (stimulable phosphor plate 51) described in Patent Document 3 has constitution wherein a stimulable phosphor plate (stimulable phosphor plate 56), in which a stimulable phosphor layer is formed on a substrate is interposed between two moisture-proof protective films (protective layer films 57 and 59), and the peripheral areas of each moisture-proof protective film is fused by a heat-sealing method. Owing to this constitution, the stimulable phosphor plate is completely sealed by the moisture-proof protective films, to prevent the stimulable phosphor layer from becoming moist (Paragraph numbers 0028-0032, see FIG. 1).
However, in the case of the radiation image conversion panel disclosed by Patent Document 3, if a stimulable phosphor plate composed of a substrate and a stimulable phosphor layer has a thickness that is not less than the prescribed thickness, there is a possibility, when fusing each peripheral area of the moisture-proof protective film that seals the stimulable phosphor plate, that creases and waves may be formed on the moisture-proof protective film in the direction from the fused portion of each moisture-proof protective film toward the stimulable phosphor plate. In the present specification, “creases” on the moisture-proof protective film mean linear folds caused on the surface of the moisture-proof protective film, while, “waves” on the moisture-proof protective film means a gentle undulation caused on a surface of the moisture-proof protective film by difference in tension resulting from unbalanced stretching of the moisture-proof protective film.
For example, when creases and waves on the moisture-proof protective film as in the foregoing reach the radiation detection area on the stimulable phosphor plate, linear streaks and unevenness of the images may appear on the obtained radiation image, in addition to images resulting from the patient. In particular, these image defects tend to appear remarkably, when using a laser beam of excellent beam convergence to excite the stimulable phosphor layer of the stimulable phosphor plate.
On the other hand, when a crease is caused on each of both moisture-proof protective films, moisture-proof ability for the stimulable phosphor layer of the stimulable phosphor plate may be deteriorated, and quality of the stimulable phosphor plate itself may have a problem, and when generation of waves on the moisture-proof protective film is remarkable, flatness of the radiation detection surface of the stimulable phosphor plate may also be deteriorate.
As a stimulable phosphor layer of the radiation image conversion panel used for the radiation image conversion method, there is a method to use a stimulable phosphor layer composed of a microscopic pseudo-columnar block that is formed by depositing stimulable phosphor on a substrate having a microscopic uneven pattern, such as the one, for example, employed in JP-A No. 61-142497.
Further proposed are a method to use a radiation image conversion panel having a stimulable phosphor layer wherein a crack between columnar blocks each being obtained by depositing stimulable phosphor on a substrate having a microscopic pattern as described in JP-A No. No. 61-142500, is further developed; a method to use a radiation image conversion panel wherein a stimulable phosphor layer formed on the surface of a substrate is cracked from its surface side to be pseudo-columnar, as that described in JP-A No. 62-39737; and further a method to provide a crack by forming a stimulable phosphor layer having cavities on an upper surface of a substrate through vacuum evaporation, and then, by making the cavities to grow, as that described in JP-A No. 62-110200.
In JP-A No. 2-58000, there is further proposed a radiation image conversion panel having a stimulable phosphor layer, in which a slender column crystal that is at a certain angle to the direction of a normal line on a substrate, is formed on the substrate by a vapor deposition method.
In a trial to control the form of these stimulable phosphor layers, it is possible to restrain diffusion of stimulable excited light (or photo-stimulated luminenscence) to the lateral direction (arriving at the surface of the substrate after repeating reflection on the interface of cracks (column crystal)), by making all of the stimulable phosphor layers to be columnar, thus, it is possible to remarkably improve the sharpness of images formed by photo-stimulated luminescence, which is a special feature.
However, it is commonly known that most stimulable phosphors are highly hygroscopic, and if they are left under general environmental conditions, they gradually absorb moisture in the air, and capacities are considerably deteriorated with a lapse of time, in the radiation image conversion panel having these stimulable phosphor layers formed by a vapor growth (deposition) method.
There has been employed a method to prevent moisture absorption of stimulable phosphor layers by forming a barrier by using a moisture-proof protective film on which a thin film of metal oxide or of silicon nitride is deposited, and by sealing the stimulable phosphor layer formed by dispersing europium-activated alkaline earth metal fluoride halide type phosphor particles in a binder, as described, for example, in Patent Document 4.
Further, with respect to protection against water vapor for moisture-absorbing phosphor, Patent Document 5, for example, discloses an example to use a laminated film wherein a polyparaxylene film and a moisture-proof film such as silica are formed in succession by a CVD method, for protecting a phosphor, such as CsI representing a scintillator material, against water vapor.
However, a stimulable phosphor crystal formed by the vapor deposition method is more hygroscopic than a stimulable phosphor layer formed by dispersing europium-activated alkaline earth metal fluoride halide phosphor particles in a binder, and also more than a phosphor such as CsI representing a scintillator material, and there is no protection by the binder for stimulable phosphor crystal made by a vapor deposition method, thus, protection against water vapor is more important and a method for total prevention of moisture adsorption has been desired.
(Patent Document 1) JP-A No. 10-82899
(Patent Document 2) JP-A No. 2002-122698
(Patent Document 3) JP-A No. 2000-171597
(Patent Document 4) JP-A No. 11-344598
(Patent Document 5) JP-A No. 2001-235548